Saline Water Intrusion into the Coastal Aquifers of the Periyar River Basin, Central Kerala, India

Abstract

Successful management of coastal groundwater resources depends largely on the accurate assessment and prediction of aquifer behaviour and the saline water–freshwater interface both under the natural and the anthropogenic conditions. The prime objective of this paper is to assess the extent of saline water intrusion into an urban groundwater aquifer and to know the impact of this phenomenon on the quality of groundwater. For this purpose, the coastal belt of Kochi near where the Periyar River debouches into the Arabian Sea is selected, considering the gravity of the problem prevailing in this region. The Periyar River is the longest river and the river with the largest discharge potential in the state. It is one of the few perennial rivers in the region and provides drinking water for several major towns of Central Kerala. The coastal zone of the Periyar River Basin is one of the most densely populated areas in the country. Geologically, the area has three distinct formations, viz., the crystalline rocks of the Precambrian age overlain by the Tertiaries and the Recent alluvium. The analysis of apparent resistivity variation with change in electrode spacing helped to deduce the depth and resistivity distribution of various subsurface units which in turn are interpreted in terms of various geological formations. The resistivity profiling coupled with resistivity sounding was found to be a highly effective method for determining the freshwater areas and the saline water contaminated zones at different depth levels. The electrical resistivity curves obtained expressed a dominant trend of decreasing resistivity with depth (thus increasing salinity). The rock matrix, salinity and water saturation are the major factors controlling the resistivity of the formation. Moreover, the fresh and saline groundwater inter-phase has been investigated, and it is observed that the saline water intrusion into the aquifers can be accurately mapped using surface DC resistivity method. The iso-apparent resistivity map shows that contour patterns are almost parallel to the coast (north–south trend), with the increasing values towards east. The apparent resistivity along the eastern side is comparatively high when compared with the western margin of the area, which is an indication that the salinity is decreasing towards the eastern side. It is also observed that the resistivity values decrease with increase in depth of investigation. The very low resistivity is an indication that the aquifers are of poor chemical quality. Based on the analysis of the above observations, the freshwater saline water interface of the study area is demarcated.